Current fiber coating techniques apply a soft inner coat material to protect against micro-cracks and a rigid outer layer to offer handling protection. In the fabrication of fiber optic gyroscope sensing coils, a third adhesive layer is applied for coil bonding. Once the adhesive layer is applied, the entire sensing coil assembly is fused under heat and pressure. The process results in a single monolithic coil.
The protective and adhesive coating materials have a high coefficient of thermal expansion relative to the fiber itself. This results in the coating materials expanding and contracting to a greater degree than the fiber when exposed to temperature changes while in service or testing. The thermal expansion can therefore have an adverse effect on the performance of the sensing coil.
The fabrication of sensing coils currently utilizes two different kinds of adhesive materials, a “wet” adhesive material process and “dry” adhesive material process. The wet process provides superior strength, ruggedness, and adhesive crosslinking when compared to the dry process, yielding better physical performance of the sensing coil. The dry process, however, provides more favorable manufacturing characteristics, such as ease of handling and winding.
For example, dry adhesives, such as fused thermoplastic coating, allows a fiber to be pre-impregnated, and the pre-impregnated fiber can be wound at a later time, and set the adhesive. In contrast, wet adhesives, such as resins and epoxies, are applied to the fibers before winding and the fibers must be formed into the desired coil shape. The wet adhesive begins to set thereafter even without necessarily needing to apply heat. Therefore, wet adhesives have a limited pot life and the process affords less flexibility than the dry process, as interrupting or delaying winding process creates difficulties with wet adhesive begins to set.